1. Field of the Invention
The present invention is in the field of poly(vinyl butyral) (PVB) interlayers. Specifically, the present invention relates to the formulation of a PVB sheet where the melt viscosity may be controlled through the use of hydrogen peroxide (H2O2) included as part of the melt blend of PVB resin.
2. Description of Related Art
Generally, multiple layer glass panels refer to a laminate comprised of a polymer sheet or interlayer sandwiched between two panes of glass. The laminated multiple layer glass panels are commonly utilized in architectural window applications, in the windows of motor vehicles and airplanes, and in photovoltaic solar panels. The first two applications are commonly referred to as laminated safety glass. The main function of the interlayer in the laminated safety glass is to absorb energy resulting from impact or force applied to the glass, keep the layers of glass bonded even when the force is applied and the glass is broken, and prevent the glass from breaking up into sharp pieces. Additionally, the interlayer generally gives the glass a much higher sound insulation rating, reduces UV and/or IR light transmission, and enhances the aesthetic appeal of the associated window. In regards to the photovoltaic applications, the main function of the interlayer is to encapsulate the photovoltaic solar panels which are used to generate and supply electricity in commercial and residential applications.
The interlayer is generally produced by mixing a polymer resin such as poly(vinyl butyral) (PVB) with one or more plasticizers and melt blending or melt processing the mix into a sheet by any applicable process or method known to one of skill in the art, including, but not limited to, extrusion. Other additional additives may optionally be added for various other purposes. After the interlayer sheet is formed, it is typically collected and rolled for transportation and storage and for later use in the multiple layer glass panel, as described below.
The following offers a simplified description of the manner in which multiple layer glass panels are generally produced in combination with the interlayers. First, at least one polymer interlayer sheet is placed between two substrates and any excess interlayer is trimmed from the edges, creating an assembly. It is not uncommon for multiple polymer interlayer sheets to be placed within the two substrates creating a multiple layer glass panel with multiple polymer interlayers. Then, air is removed from the assembly by an applicable process or method known to one of skill in the art; e.g., through nip rollers, vacuum bag, vacuum ring, or another deairing mechanism. Additionally, the interlayer is partially press-bonded to the substrates by any method known to one of ordinary skill in the art. In a last step, in order to form a final unitary structure, this preliminary bonding is rendered more permanent by a high temperature and pressure lamination process known to one of ordinary skill in the art such as, but not limited to, autoclaving.
Generally, two (2) common problems are encountered in the art of manufacturing multiple layer glass panels: delamination and bubbling from inefficient de-airing or de-gassing. Delamination is the splitting or separating of the laminate into the individual layers, e.g., the separating of the substrates from the interlayer. This typically occurs around the edges of the multiple layer glass and is usually the result of the breakdown of the bond between the glass and the interlayer by atmospheric moisture attack, panel sealant degradation and/or excessive stress imposed on the glass. Certain conditions tend to accelerate the manifestation of edge delamination, especially when one or more of the substrates is wavy or warped. If the delamination extends too far into the panel, the structural integrity of the glass panel may become compromised.
De-airing or de-gassing is the removal of the presence of gas or air in a multiple layer glass panel. Gas trapped in a multiple layer glass panel can have a negative or degenerative effect on the optical clarity and adhesion of the panel. During the manufacturing process of laminated multiple layer glass panel constructs, gases can become trapped in the interstitial spaces between the substrates and the one or more polymer interlayers. Generally, this trapped air is removed in the glazing or panel manufacturing process by vacuum de-airing the construct, nipping the assembly between a pair of rollers or by some other method known to those of skill in the art. However, these technologies are not always effective in removing all of the air trapped in the interstitial spaces between the substrates, especially when one or more of the substrates is wavy or warped. Generally, the presence of a gas in the interstitial spaces of a multiple layer glass panel takes the form of bubbles in the polymer interlayer sheet(s) or pockets of gas between the polymer interlayer sheet(s) and the substrates—known as “bubbling”.
Delamination, bubbling and visual defects are particularly evident and acute when the interlayer is used in conjunction with warped or wavy glass or other applications where high flow (or low flow) may be important, including, but not limited to, tempered glass, heat strengthened/toughened glass, mismatched glass, bent glass for making windshields, and in photovoltaic applications where additional components are included that cause unevenness. For example, the processing of tempering glass creates some distortion and roller waves, and thus tempered glass is generally not as flat as ordinary annealed glass. In such applications, the waviness of the substrates creates gaps between the substrates themselves and between the substrates and the interlayer(s), resulting in an increased tendency of delamination and bubble formation. Delamination and bubble formation, or any other form of visual imperfection, are undesirable and problematic where the end-product multiple layer glass panel will be used in an application where optical quality or structural integrity is important. Thus, the creation of a near perfect laminated glass essentially free of any gaseous pockets or bubbles is paramount in the multiple layer glass panel manufacturing process. Not only is it important to create a multiple layer glass panel free of gaseous pockets and bubbles immediately after manufacturing, but permanency is also important. It is not an uncommon defect in the art of multiple layer glass panels for dissolved gases to appear (e.g., for bubbles to form) in the panel over time, especially at elevated temperatures and under certain weather conditions and sunlight exposure. More gases or excessive air will be trapped in the laminated panels if glass panels are warped and/or wavy. The excessive air trapped in the laminated panels will significantly reduce the tolerance of the panels for the elevated temperatures and adverse weather conditions, i.e., bubbles could be formed at lower temperatures. Thus, it is also important that, in addition to leaving the laminate production line free from any bubbles or gaseous cavities, the multiple layer glass panel remains gas-free for a substantial period of time under end-use conditions to fulfill its commercial role.
As a measure to prevent delamination, bubbling and other visual defects with warped glass, it has become common to either increase the flowability, the thickness, or both characteristics in the interlayer. This increases the capability of the interlayer to fill the gaps that are inherent in the use of warped glass substrates. However, there are several problems with these interlayer compositions previously utilized in the art. For example, with an increase in thickness comes an increase in cost. Additionally, increasing flow also often requires additional expense. One can increase the amount of plasticizer loading or can use a lower molecular weight PVB. Plasticizer loading creates other problems including plasticizer exudation and creep.
The surface roughness (characterized as Rz) of a sheet of PVB is generally known to those of skill in the art as the measure of the finer surface irregularities in the texture of the interlayer surface, i.e., peaks and spaces there between on the surface distinguished from the imaginary plane of the flattened polymer interlayer sheet. An appropriate level of surface roughness is needed for good de-airing performance during lamination. If the surface roughness is too low, de-airing will become impossible. On the other hand, if the surface roughness is too high, the large surface irregularities in the interlayer will be difficult to remove during lamination, resulting in the surface irregularities being visible. Either too low or too high surface roughness will result in poor de-airing performance and cause more bubbling, delamination and/or visual defects.
The degree of surface roughness is at least in part the result of the manufacturing process employed to create the interlayer. Generally, there are two ways to generate surface roughness during manufacturing: by forming “random rough” surfaces through melt fracture during extrusion (see, for example, U.S. Pat. Nos. 5,595,818 and 4,654,179, the entire disclosures of which are incorporated by reference herein), or by imparting a surface on the interlayer sheet by embossing (see, for example, U.S. Pat. No. 6,093,471, the entire disclosure of which is incorporated by reference herein). Surfaces formed by both methods (that is, both random rough and embossed surfaces) will be affected by the rheological properties (such as flow) of the interlayer. For example, an increase in flow may result in a decrease in the surface roughness formed by melt fracture during extrusion (that is, the surface roughness, Rz, may be too low, which will make de-airing more difficult, causing more bubbling, delamination or other visual defects). Again, such defects are undesirable and can result in visual and structural defects as well as decreased mechanical strength of the interlayer and the resultant multiple layer glass panel. In some extreme cases, surface roughness formed by melt fracture will be extremely low (or the sheet will be very smooth) due to the formulation changes for improving flow because there will be no ‘fracturing’ of the polymer melt to cause the surface irregularities. In such cases where there is very low or no surface roughness level, or even where increased surface roughness is desired (surface roughness levels higher than the surface roughness levels formed by melt fracture), embossing techniques have to be employed to produce a surface having a sufficient surface roughness, Rz (such as at least 25 μm, or at least 30 μm, or greater than 30 μm). The embossing process requires additional manufacturing steps and may be a more complicated process, and the end result may be lower efficiency, increased energy costs, and loss of production capacity.
Creep is the tendency of the solid interlayer material to slowly move or deform permanently under the influence of stresses causing the two layers of glass to move relative to each other. Creep can be problematic because multiple layer glass panels tend to become deformed and elongated as a result of the interlayer creep. For example, over time the two glass panels may begin to slide apart from one another as a result of the interlayer creep. Thus, with many previous attempts at increasing flow came a greater tendency for creep and the resultant deformation of the interlayer. In some situations, this creep can result in structural defects and decreased mechanical strength of the interlayer and the resultant multiple layer glass panel.
While an increase in flow can be achieved by using a PVB resin which has a lower molecular weight (Mw) to begin with to form the PVB interlayer or sheet, this solution presents a host of different problems. First, before the PVB resin is formed into a PVB sheet to be used as an interlayer, the lower molecular weight of the PVB resin can make the manufacturing process significantly more difficult due to handling issues with the lower molecular weight PVB resin.
Further, in order to generate lower molecular weight PVB resin, different starting materials generally had to be used. For example, poly(vinyl alcohol) (PVOH) grades having differing molecular weights and mixtures of resins previously had to be used to make the PVB resin having a particular target molecular weight. This process can be difficult and time consuming. Further, a particular molecular weight formulation (starting PVOH) would always have a fixed flowability in the resultant PVB sheet. Thus, suppliers of PVB sheets would be required to stock a variety of different molecular weights of PVOH in order to make a variety of PVB resins (having different molecular weights) that they, in turn, would need to provide a variety of different formulations of PVB sheet depending on what a customer needed.
This latter problem is particularly important. While it was previously possible to provide PVB resins with a variety of different molecular weights (and thus PVB sheets with various levels of flowability), the range of such products was limited by the available range of inputs. Thus, if a manufacturer was limited in the number of PVOH grades (different molecular weights) they had available, they were limited in the number of PVB resin formulations they could supply, and, thus, the available flowability options for the resultant PVB sheet based on the types of PVOH available and the acceptable and available plasticizer load. It should be recognized that while the present disclosure discusses the usefulness of higher flow PVB, in other situations lower flow PVB formulation are desired and, therefore, it is desirable in the art to be able to produce a PVB sheet having particular flow characteristics (such as high flow or low flow), not just to produce sheets with higher flow and/or the lowest melt viscosity available. Stated differently, it is desirable to be able to vary the flow and molecular weight of PVB resin to be able to produce PVB sheet having different physical properties as desired or needed.
Summarized, visual defects such as bubbling and delamination, as well as other defects, are common problems in the field of multiple layer glass panels which are particularly acute when using wavy or warped substrates. In an attempt to correct these problems associated with wavy or warped substrates, it became common to use an interlayer with an increased thickness or flow or both. However, increased thickness and/or flow of previously utilized interlayers created a host of new problems and unfavorable sacrifices, including, but not limited to, increased manufacturing costs (i.e., the costs associated with an increased thickness in the interlayer) and the requirement to utilize different PVOH starting materials to generate lower molecular weight PVB resins that could be used to control the physical properties of the PVB sheet.